In accessing the tops of rolling stock (tank trucks, tank railroad cars, etc.), a gangway or bridge is used to allow workers to move from an elevated loading platform to the top of the rolling stock. Such a gangway often includes self-leveling steps and is pivotally mounted to a platform that is disposed above the top of the rolling stock. In one type of these gangways such as disclosed in U.S. Pat. No. 7,140,467, which is hereby incorporated herein in its entirety by this reference for all purposes, the gangway is a retractable stairway 26 that has a retracted disposition in which the free end, i. e., the distal end, of the stairway is stored in a position that resides above the walking surface of the floor 22 to which the gangway is pivotally mounted. This retractable stairway 26 can include an adjustable chain stop 27 that is configured and disposed so as to limit downward swinging movement of the stairway 26.
In gangways that are disclosed in U.S. Pat. Nos. 4,679,657 and 7,216,741, which patents are hereby incorporated herein in their entirety by this reference for all purposes, the gangway can be moved pivotally like a drawbridge between a relatively elevated stowed orientation with respect to the top of rolling stock and a relatively lowered operative orientation with respect to the top of rolling stock. These gangways can include expansion springs or hydraulic actuators that are interconnected so as to bias or counterbalance the gangway to remain in any given orientation, for example the relatively elevated stowed orientation, and the user can push against this biasing or counterbalancing force to dispose the gangway in a different orientation, for example the relatively lowered operative orientation that leads to the top of the rolling stock. In such embodiments, the expansion springs and/or hydraulic actuators serve to counterbalance the weight of the gangway and so enable an individual user to move the gangway between the lowered operative orientation and the elevated stowed orientation by the exertion of a reasonable level of manual force by a typical individual user.
In gangways that are disclosed in U.S. Pat. Nos. 8,015,647 and 8,403,109, which patents are hereby incorporated herein in their entirety by this reference for all purposes, the gangway can be held in the lowered operative orientation by extending the piston rods from within actuating cylinders that force the gangway downward by overcoming the biasing force of expansion springs. However, such actuating cylinders are an added expense in themselves as well as introducing the further complexity and expense of providing the pressurized fluid and associated motors, compressor, valves and controls and that are necessary to the actuate the cylinders in order to operate them. This patent also discloses a foot-activated lock that can be engaged so as to restrain the gangway in the lowered orientation. This foot-activated lock is disposed near the walking surface of the platform from which the gangway is pivotally attached. Unfortunately, the heavy foot traffic of workers between the platform and the gangway can result in an inadvertent deactivation of the lock, and once the lock is deactivated the gangway 30 can be moved away from the top of the rolling stock and expose workers remaining on the rolling stock to a risk of falling if they are unaware of the removal of the gangway.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate at least one presently preferred embodiment of the invention as well as some alternative embodiments. These drawings, together with the written description, serve to explain the principles of the invention but by no means are intended to be exhaustive of all of the possible manifestations of the invention.